The present invention relates to a single-shaft centrifugal compressor which is used as a gas compressor in various industrial plants such as petroleum refining plants, petrochemical plants and fertilizer synthesizing plants, as well as for the purpose of transporting natural gas in natural gas transporting pipeline. More particularly, the invention is concerned with a single-shaft multi-stage centrifugal compressor having an intermediate suction line and improved in such a manner as to protect shaft seals and other parts of the compressor from any corrosive or toxic component which may be contained by the gas (referred to as "intermediate suction gas", hereinafter) introduced through the intermediate suction line.
In general, a single-shaft multi-stage centrifugal compressor has a plurality of impellers carried by a shaft provided in a casing. The shaft is provided at its both ends with shaft seals.
The shaft seals are supplied not only with a seal oil but also with a part of the gas discharged from the compressor as a seal gas flow for the purpose of preventing the seal oil from coming into the compressor. Thus, the seal gas is a mixture of the intermediate suction gas (gas introduced from the recycle line) and the gas from the main gas line.
The seal gas and the seal oil after the use are separated from each other by a drainer and are discharged to the outside.
In another known sealing method, in order to positively form a seal gas flow, a clean buffer gas (seal gas) having a required sealing pressure is directly supplied from an external supply to the shaft seals by a suitable pressure-differential control.
The detail of the single-shaft multi-stage compressor is disclosed, for example, in an article which is entitled "The Use of Centrifugal Compressors in Ammonia Production Plants" which is a technical report of QUADERNI PIGNONE 9.
The above-described compressor which employs the sealing gas composed of the intermediate suction gas and the gas from the main line inevitably suffers from the following problem. Namely, in this type of compressor, part of the mixed gas as the seal gas is allowed to flow into the shaft seals and the drainer due to balance of pressures. Usually, the intermediate suction gas collected from a chemical process contains corrosive components such as carbonate. As the result, the shaft seals and the drainer are corroded and, therefore, cannot stand a long use.
On the other hand, the sealing method which relies upon the external supply of buffer gas results in a raised installation and running costs due to the necessity for a separate source for supplying the pressurized buffer gas.